1. Field of the Invention
The invention is directed to an apparatus for the electrical function testing of wiring matrices, particularly of printed circuit boards, that comprise a plurality of first measuring locations arranged at a relatively great distance from one another and at least one group of second measuring locations arranged at a finer spacing from one another.
2. Description of the Related Art
In automatic testing units and testing adapters for unequipped and equipped printed circuit boards as well as for wiring matrices using solder or crimp techniques, the contacting of the selected measuring locations is usually undertaken via resilient test probes. The resilient test probes arranged in accordance with the grid dimension of a wiring matrix to be tested are secured with spring sleeves that are pressed into a carrier plate and into which the test probes are inserted. What are usually decisive in the selection of the test probes are the smallest distance of the measuring locations from one another as well as the current load across the diameter of the resilient test probes, whereby, however, 0.38 mm is cited as the lower limit dimension for the diameter (Productronic, May 1989, pages 36-40).
Conductivity and insulation measurements are implemented between the test locations formed by the grid points in accord with the layout, being implemented with the known apparatus for electrical function testing of printed circuit boards. Since the resilient test probes provided for contacting the test locations must be arranged in the grid of the printed circuit board, the realization of such apparatus increasingly encounters fundamental difficulties given a decreasing grid dimension of the printed circuit boards and areas of the printed circuit boards that are becoming larger. Thus, an arrangement of the resilient test probes in grid dimensions below 1 millimeter can hardly be governed in terms of precision mechanics given a reliable, mechanical contacting of the test locations. The number of leads and switch elements required also increases with the number of measuring locations which, for example, can amount to a hundred thousand, as a result whereof considerable apparatus-oriented outlay and correspondingly high costs are incurred. Moreover, the probability of a complete contacting of the printed circuit boards decreases considerably with the number of measuring locations.
EP-B-0 102 565 discloses an apparatus for the electrical functioning testing of wiring matrices wherein the previous standard ohmic contacting of the measuring locations is replaced by a non-touching, ionic contacting via gas discharge paths. To this end, a plurality of gas discharge channels provided with electrodes is introduced into a carrier plate that can be put in place on the wiring matrices, whereby the gas discharge channels arranged in the grid of the wiring matrices are open toward the measuring locations. When two selected measuring locations are electrically conductively connected to one another, for example by an interconnect, then the allocated gas discharge channels form two series-connected gas discharge paths that can be ignited by applying an adequately high voltage to the electrodes. A current transport that can be evaluated for testing purposes ensues with the ignition of the gas discharges. When the ignition of the gas discharges fails to occur or when an excessively low current flows after an ignition, then conclusions can be drawn regarding an interrupted connection or an electrically conductive connection that did not exist from the very outset between the selected measuring locations. When an AC voltage is overlaid on the voltage applied to the electrodes, then the change in current resulting therefrom can be measured phase-sensitive vis-a-vis the applied AC voltage and can be utilized for calculating the resistance of a connection existing between the selected measuring locations. The known apparatus thus enables conductivity and insulation measurements, whereby an extremely high reliability is achieved by avoiding ohmic contacts. In particular, wiring matrices having small grid dimensions of the measuring locations down to 0.1 mm can then be reliably tested with the principle of ionic contacting of the measuring locations via gas discharge channels that can be realized with extremely small dimensions.
In the apparatus disclosed by EP-B-0 102 565, thus, a separate gas discharge channel in the carrier plate is provided for every measuring location. This is particularly necessary when different wiring matrices are to be tested with the same apparatus.
There are, however, specific wiring matrices or printed circuit boards or sub-regions thereof that, for example, are intended for direct contacting of housing-free, integrated circuits and that have the following, characteristic structure. They contain regions of measuring locations having a spacing from one another that is even smaller than the spacing among the other measuring locations that are arranged at the standard spacing or, respectively, grid from one another. This slight spacing, for example, coincides with the connection grid of the afore-mentioned, housing-free, integrated circuits. Given such wiring matrices or printed circuit boards, exactly one conductive connection typically exists between a measuring location of the fine region and a measuring location of the region referred to here as the coarse region of the remaining measuring locations. The films of the MIKROPACK format for SMD-ICs of Siemens AG, Berlin and Munich, Germany, can be cited here as an example, the spacing of the measuring locations in the fine region therein being potentially smaller than 100 .mu.m.
When the grid dimension of the measuring locations in the fine region falls below 100 .mu.m, then the reliable contacting of these measuring locations via allocated gas discharge channels can present difficulties. Over and above this, the demands made of the adjustment precision of the carrier plates relative to the wiring matrix increase, this having to be better than 10 .mu.m given a grid spacing of 100 .mu.m. This leads either to a longer testing time or to higher manufacturing costs for the apparatus utilized for the function testing.